Playing in his second rehab game for Triple-A Pawtucket on Sunday, Kotsay aggravated his right calf muscle while running the bases. The left-handed-hitting outfielder also had trouble with the calf a couple of weeks back, though the Red Sox don't think it is as serious this time.
Still, Kotsay will temporarily halt his Minor League assignment and work with the Red Sox's training staff in Boston for the time being.
"He hit a ground ball yesterday in his second at-bat," Red Sox manager Terry Francona said. "In his words, he was trying to shift into another gear. He felt it. Fortunately, he pulled up a little bit. I talked to him about an hour ago -- he's going to get treatment. That's all he'll do today."
The medical staff will reevaluate Kotsay on Tuesday, and Francona will confer with the outfielder-first baseman when the team returns to Fenway on Wednesday.
"It's a setback," Francona said. "He didn't do it like he did it in Florida, but it is a setback. He was a little frustrated. I can understand. But it's not to the point where it was last time, which is good."
Rocco Baldelli, who is coming back from a left hamstring strain, started his rehab at Pawtucket on Monday, leading off and serving as the designated hitter.
Right-hander Daisuke Matsuzaka will start his rehab assignment on Tuesday at Pawtucket, throwing roughly three innings.
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.